1. Field of the Invention
This invention relates to a magnetooptical recording method having high recording sensitivity and especially relates a conventional or non-overwritable method.
2. Related Background Art
In recent years, many efforts have been made to develop an optical recording/reproduction method which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed, and a recording apparatus, a reproduction apparatus, and a recording medium used therefor.
Of various optical recording/reproduction methods, a magnetooptical recording/reproduction method is most attractive due to its unique advantages. That is, it is repetitively possible to erase recorded information, and to record new information after previous information is erased.
A recording medium used in the magnetooptical recording/reproduction method employs a magnetic thin film having a perpendicular magnetic anisotropy as a recording layer. A typical magnetic thin film comprises an amorphous heavy rare earth-transition metal alloy. Typical alloys are GdFe or GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like.
The recording layer normally has tracks formed by concentrical or spiral grooves or ridges, and information is recorded on the tracks.
Information to be recorded is binarized in advance, and is expressed by two small magnetic domains, i.e., a small magnetic domain (called a pit, bit, or mark) having an upward direction of magnetization, and a small magnetic domain having a downward direction of magnetization. In this specification, following the latest example, the former domain will be referred to as a mark B.sub.1 hereinafter, and the latter domain will be referred to as a mark B.sub.0 hereinafter. These marks B.sub.1 and B.sub.0 respectively correspond to one and the other of digital signal levels "0" and "1".
In general, the direction of magnetization of the recording layer is aligned in an upward or downward direction upon application of a strong external field after the manufacture of a medium, i.e., before recording. This process will be referred to as an initialize process hereinafter.
The reasons why the initialize process is to be executed are that 1 the direction of magnetization of a magnetic thin film is not uniform in a film formation state, and that 2 since it is difficult to change (modulate) the direction of a bias field Hb necessary for forming a bit at high speed in actual recording, only one mark (mark B.sub.1) is to be formed according to information.
Before recording, the direction of magnetization of the entire magnetic layer is aligned in one of upward and downward directions (corresponding to the direction of the mark B.sub.0), and thereafter, the mark B.sub.1 having an opposite direction of magnetization is intermittently formed according to binary information.
Information is expressed by the presence/absence of the mark B.sub.1 and/or a mark length. This is the actual recording method.
The mark is formed by using a laser beam which is focused to have a spot size as small as about 1 micron, and a bias field Hb. More specifically, upon radiation of a laser beam, the irradiated portion of the magnetic thin film is heated to a temperature equal to or higher than a Curie temperature, thereby making a coercivity of the irradiated portion zero. In this state, when the radiation of the laser beam is stopped, or the irradiated portion is separated from the beam, the temperature of the irradiated portion is naturally decreased. When the temperature of the irradiated portion is decreased below the Curie temperature, the coercivity appears again in the irradiated portion. At this time, if a bias field Hb is present, the direction of magnetization follows the direction of Hb. In this manner, the mark B.sub.1 having the same direction of magnetization as the direction of Hb is formed.
The formed mark B.sub.1 is detected by utilizing a magnetooptical effect (Kerr effect or Faraday effect), thereby reproducing recorded information.
A C/N ratio has a positive correlation with 1 .theta.k and 2 the intensity of a laser beam to be radiated in a reproduction mode. Thus, in order to increase the C/N ratio, a magnetic material having large 1 .theta.k must be selected. In this case, a magnetic material having large .theta.k has a high Curie temperature without exceptions. In order to increase the C/N ratio, if 2 the intensity of a laser beam to be radiated in a reproduction mode is increased, the temperature of the recording layer undesirably exceeds the Curie temperature, and as a result, information is lost. For this reason, a magnetic material whose Curie temperature is as high as possible must be selected.
However, in the conventional recording method, as described above, the recording layer must be heated up to a temperature equal to or higher than the Curie temperature to make the coercivity zero. Therefore, when a magnetic material having a high Curie temperature is selected, recording sensitivity is undesirably lowered. That is, a high-intensity laser beam source is required, or a recording linear velocity must be lowered.
In other words, in the conventional recording method, when recording sensitivity is to be increased, a magnetic material having a low Curie temperature must be selected, and hence, the C/N ratio is lowered, thus posing the first problem.
On the other hand, a conventional magnetooptical recording method and recording apparatus require a bias field Hb apply means (more specifically, a permanent magnet or an electromagnet) as well as the laser beam source.
Since a magnetic field is abruptly lowered when it is separated from a magnet, the above-mentioned two means always occupy close positions in the recording apparatus. For this reason, the degree of freedom of design for an arrangement of members in the recording apparatus is lowered, thus posing the second problem.
The second problem often leads to a secondary problem. That is, it is difficult to attain a compact recording apparatus.